1. Field of the Invention
The present invention relates to hydraulic systems that operate actuators, such as cylinder-piston arrangements, and more particularly to valve assemblies that control flow of fluid to and from the actuators.
2. Description of the Related Art
A wide variety of machines are operated by a hydraulic system that has one or more hydraulic actuators, such as piston-cylinder arrangements or hydraulic motors, which move components on the machine. A separate valve assembly controls the flow of pressurized fluid from a pump to each hydraulic actuator and the return of that fluid to a reservoir tank. One common type of valve assembly has a spool that is moved in a bore by pilot pressure selectively applied to surfaces at opposite ends of the spool. The spool has annular notches that, in different positions of the spool, provide paths between various passages which open into the bore and which connect to the hydraulic actuator, the pump, and the reservoir tank. Some of those paths have variable control, or metering, orifices through which the fluid flows.
The speed of the hydraulic actuator depends on the cross-sectional area and the pressure drop across those variable control orifices. To facilitate control of the actuator, pressure compensating devices have been designed to set and maintain the pressure drop. The result is a self-adjusting system that provides a substantially constant pressure drop across the control orifice. Therefore, with the pressure drop being held constant, the speed of the hydraulic actuator is determined only by the cross sectional area of the control orifice that is varied by the machine operator.
For some types of equipment, such as implements attached to the hitch on an agricultural tractor, it is desirable also to provide pressure compensation in the tank return fluid path through the valve assembly. For example, some agricultural implements place a relatively large gravitational load on the hydraulic actuator of the tractor hitch. Thus lowering the implement can take advantage of the resultant force to drive fluid out of the actuator to the tank and that return flow can be controlled to adjust the rate at which the implement lowers.